Hydraulic fracturing is a stimulation treatment which consists of propagating fractures in rock layers by the introduction of pressurized treatment fluid. The treatment fluid is pumped at high pressure into the hydrocarbon bearing area of a wellbore that extends into the target reservoir. The high pressure fluid when introduced to the wellbore causes cracks or fractures which extend back and away from the wellbore into the surrounding rock formation.
Depending on the nature of the reservoir and the particular rock formation, acid, chemicals, sand or other proppants are selectively mixed into the treatment fluid to improve or enhance the recovery of hydrocarbons within the formation.
There have been a number of recent developments with respect to wellbore treatment tools including the development of fracturing strings for staged well treatment. Such fracturing strings are predicated on creating a series of isolated zones within a wellbore using packers. Within each zone there are one or more fluid ports that can be selectively opened from the surface by the operator. A common mechanism comprises a sliding sub actuated by a ball and seat system, the movement of which is used to open fluid ports. By sizing the seats and balls in a complimentary manner, increasingly larger balls may be used to selectively activate a particular sliding sub allowing the operator to stimulate specific target areas.
Further development and refinement has resulted in fracturing strings having multiple fluid ports within each isolated zone. The seats and balls are sized such that one ball may be used to actuate a series of sliding subs within an isolated zone or a series of sliding subs in different isolated zones. This is achieved using seats that expand or deform to allow the ball to pass. The ball is deployed from the surface and it travels down the well bore becoming lodged on the deformable seat forming a temporary seal. The fluid pressure on the ball and seat actuates the sliding sub into its second position, in the process opening the fluid port. The seat eventually deforms allowing the ball to pass and the ball moves down to the next sliding sub which it actuates in the same manner. The last or lowest seat in the isolated zone is sized such that the ball will not pass and thus forms a seal preventing the flow of treatment fluid to lower zones that may have already been actuated. The use of multiple fluid ports allows multiple stages within the isolated zone to be stimulated with one surface treatment.
When using a fracturing string using multiple deformable seats and a single ball, as described above, the user may encounter difficulties in fracturing the lower regions of the formation within the isolated zone. The reason is that the seats are designed so that greater fluid pressure is needed to push the ball past the lower situated seats than the higher situated seats. Such greater fluid pressure, however, may be sufficient to force the fluid from the string into the wellbore and fracture the formation surrounding the already opened higher fluid ports. This results in a loss of fluid which is counterproductive to increasing fluid pressure in the fracturing string. Accordingly, the user may be unable to achieve sufficient fluid pressure to push the ball past the seats and actuate the subs situated in the lower regions of the formation. Even if the user can achieve sufficient pressure to activate the subs in the lower regions of the formation, the pressure may still be suboptimal for stimulating the lower regions of the formation. Prior art solutions have enjoyed limited success and are relatively complicated.
What is needed is a tool, and a method of using the same, for preventing the escape of treatment fluid from fluid ports within an isolated zone of a fracturing string until the treatment fluid pressure has been raised to the level required for hydraulic fracturing. This would better ensure that all fluid ports within an isolated zone can be opened and provide for more effective stimulation of the surrounding formation throughout the isolated zone.